Calcium and magnesium carbonates are usually found in close association with the calcium phosphate in the phosphate rock. During processing with sulfuric acid to produce phosphoric acid and diammonium phosphate from the phosphate rock, these carbonates substantially contribute to processing problems and degrade product quality. The carbonates cause excessive foaming by releasing carbon dioxide during acidulation treatment of the rock, Calcium carbonate is converted to gypsum which further increases the amount of waste generated and increases the consumption of sulfuric acid, Magnesium carbonate causes more serious problems during acidulation, It forms an impurity in the dilute phosphoric acid that degrades both the phosphoric acid and diammonium phosphate products. It modifies the crystallization of gypsum such that the filtration of the acid product from the gypsum waste becomes much more complicated. It also increases the consumption of sulfuric acid.
The state of the art for avoiding the problems caused by magnesium carbonate and to a lesser extent calcium carbonate impurities in phosphate rock rely on the following approaches. Rock having an undesirable level of magnesium carbonate (&gt;1% MgO basis) tends to be avoided during mining. Floatation type of beneficiation is used to concentrate the bone phosphate levels (BPL refers to tricalcium phosphate) in the rock while separating clay, sand and to much lesser extent the calcium and magnesium carbonates. Floatation separation selectivity between the carbonates and phosphates is limited. In order to keep the carbonate content in the phosphate rock to manageable levels, a large loss of phosphate values in the ore is accepted as part of a tradeoff to gain a low magnesia and high bone phosphate level feedstock for acidulation processing.
In the wet process for producing phosphoric acid, phosphate rock is digested with concentrated sulfuric acid. The resulting phosphoric acid contains metal impurities in which it may be desirable to remove or reduce these metal concentrations before the phosphoric acid is converted into fertilizers. Magnesium is a particularly troubling contaminant. High concentrations of magnesium in phosphoric acid prohibit the formulation of various grades of fertilizer which are necessary for agricultural crops.
The magnesium problem is becoming acute because a large part of the remaining phosphate rock reserves in the United States, as well as in other countries, contains unacceptably high levels of magnesium. If such phosphate rock is used to prepare phosphoric acid, the acid will contain so much magnesium that it cannot be used for high grade fertilizers. One expedient has been to blend a high magnesium phosphate rock with phosphate rock of lower magnesium content, but this is a limited and temporary answer to the problem.